Photomechanical method of producing grounded printed circuits

ABSTRACT

A photomechanical method of producing a grounded printed circuit utilizes an ungrounded circuit negative, a base film and a diffuser sandwiched therebetween to produce a negative mask having a spread image thereon. This negative mask is used in conjunction with the original ungrounded circuit negative to produce a composite positive which contains the required ground plane and upon which any further terminal grounding can be made. The resultant grounded composite negative is utilized to form the grounded printed circuit.

[ Nov. 27, 1973 United States Patent [191 Bemis m. mm mmnm um mn m fla .na m wbb h um e II. FGCK 9002 2777 9999 WMMW 5656 9206 3 .3 5809 20 6 2 333 OTHER PUBLICATIONS Bulletin for the Graphics Arts, No. 8, Eastman Kodak, 1946.

[75 Stuart A. Bemis, Winchester, Mass.

The United States of America as [73] Assignee:

represented by the Secretary of the United States Air Force,

washl-ngton Primary Examiner-Charles L. Bowers, Jr. Dec. 14, 1971 Attorney-Harry A. Herbert, Jr. et al.

[22] Filed:

[21 Appl. No.: 207,901

I ABSTRACT 9656.2, 96/27 B 96/27 R, A photomechanical method of producing a grounded 96/42, 96/43, 96/44, 29/624, 29/625 printed circuit utilizes an ungrounded circuit negative, a base film and a diffuser sandwiched therebetween to produce a negative mask having a spread image 96/41, [51] Int.

G03c 5/00 29/624, 625; 96/44, 96/41, 42, 43, 27 E, 27 R, 36, 36.2

[58] Field of Search thereon. This negative mask is used in conjunction with the original ungrounded circuit negative to produce a composite positive which contains the required ground plane and upon which any further terminal [56] References Cited UNITED STATES PATENTS grounding can be made. The resultant grounded com- 1,967,057 7/1934 Irvine.............. 3,423,205 H1969 Skaggs etall.. 2,752,245 6/1956 Hough et a1 Johnston et Nov. 27, 1973 United States Patent [191 Bemis INVENTOR .477'W/VIXS' FIE PAIENIEDNnv 27 I973 sum 2 OF 2 PHOTOMECHANICAL METHOD OF .PRODUCING GROUNDED PRINTED CIRCUITS BACKGROUND OF THE INVENTION This invention relates generally to a method of mak ing printed circuits, and more particularly to a photomechanical method of producing grounded printed circuits. i

Printed circuits make economical mass production possible, save space and weight, and increase reliability of electronic equipment. They are used in practically all types of electronic equipment, such as radio and TV sets; electrical wiring behind the dashboard in automobiles; guided-missile and airborne electronic equipment; computers; and'industrial control equipment.

The rapid adoption of the graphic art processes by the electronics industry is a demonstration of the effectiveness of those processes in achieving cost reduction and equipment miniaturization. These printed circuits are of interest to industry for the following reasons:

1. Printed circuits are the common denominator for almost all approaches to the mechanized fabrication of electronic equipment.

2. Use of printed circuits has greatly reduced the labor required for the wiring of an electronic circuit.

3. Uniformity of printed circuits improves the quality of the product through simplification of quality control.

4. Printed circuitry has helped to minimize one major cause of unreliability in electronic equipment by permitting the use of dip-soldering processes.

A common method of producing a printed circuit is set forth hereinbelow. A schematic draftsman, working from a sketch prepared by an engineer, uses precut or preprinted press-on symbols and tapes to lay out the circuit on a sheet of matte mylar. Although these circuit masters can be doneat the same size as the required finished circuit, they are more often done at a larger size 2:'l, 4:1, 5:1, :1 and when extreme accuracies are required these ratiosare apt to increase considerably. After checking by the engineer, the circuit master is sent to a photo laboratory where it is reduced to the desired finished size on a lithographic film by use of a process camera. The resulting negative is sent to the etching shop. I-Iere, usually in a .vacuum system, the negative is placed in intimate contact with a copper-clad plastic board which has been coated with a photo resist. The negative and board are then exposed to an ultra-violet light source which hardens the resist wherever the clear areas appear on the negative. The unhardened resist (the areas under the black parts of the negative) is then developed away, leaving etchant-resistant copper where the circuit terminals and wires are, and unprotected copper in the other areas. The board is then etched, removing the unprotected copper. Finally, a dipping in a resist solvent removes the hardened resist, leaving a plastic board with copper wiring, and terminals on which to mount components.

Two recent developments have made the preparation of printed circuits more difficult for the schematics draftsman. First, it was discovered that cramping" the circuit area as much as possible measurably increases the speed of the circuit operations. Second, it was found that by individually grounding the sub-circuits and terminals, the impedence and noise of the circuit is kept low. In high frequency circuits, it prevents the interference of one signal with another and, in general,

' improves the overall quality of the circuits. Thus, the

draftsman now not only has to lay out the circuit in a much smaller area but also has to surround the terminal subcircuits and wires with tape so that a ground plane is available on the final product. This is a tedious and tiring job that requires more than ordinary skill because the tapes must be held at specific distances from the terminals and wires for both performance and appearance sake. Errors due to lapses in concentration often occur during this taping process, not to mention the time required for such a task. It is not uncommon for a draftsman to spend a full day inserting a ground plane in this manner.

' SUMMARY OF THE INVENTION The method of the instant invention overcomes the problems set forth in detail hereinabove by (l substantially reducing the time required to create the grounding plane of a printed circuit, and (2) producing a more desirable printed circuit.

Producing printed circuits with the photomechanical method of this invention reduces the time required to create the ground plane to approximately forty-five minutes. The printed circuit master is prepared in a conventional manner by a draftsman, omitting the step of taping in any ground plane. Using any suitable process camera the printed circuit master is reduced to its final desired size. The resulting negative is opaqued to remove any pinholes therein after it has thoroughly dried. In the method of the instant'invention the negative is now turned upside down in order to print through the base and is punched for two hole registration using any conventional system whose pins can be utilized in a vacuum frame. A double matte sheet of Mylar frosted on both sides and an unexposed piece of film are also punched in the same manner as the negative. These three sheets, the negative, the Mylar and the unexposed film are pinned together and exposed with an over exposure factor of between 5 l0 times. The exposed film is then developed in a conventional infectious type lithographic developer in order to spread the image. The control of the spreading of the image is regulated by a systematic inspection. Under normal conditions the image is permitted to spread until an average line is approximately three times its normal width. This resulting negative or mask is now opaqued to remove any pinholes therein.

In the next step in the method of this invention, the original negative and the mask are now contacted, one at a time onto an unexposed piece of film that has been punched for composite registration therewith. This contacting process takes place with the negatives in an emulsion-to-emuision contact. The film is then developed normally in any conventional lithographic developer. This resultant composite positive now contains the circuit and a ground plane. The terminals that now require grounding may be easily grounded through the use of either liquid opaque or tape by either the engineer, draftsman or camera man working from either verbal instructions or schematic. The composite positive is now printed back to negative in order to prevent damage to the opaquing or to prevent the tape from being removed. The negative is now ready to be sent out for etching of a final printed circuit by the conventional etching procedures set forth hereinabove.

It is therefore an object of this invention to provide a photomechanical method of producing a grounded printed circuit.

It is another object of this invention to provide a photomechanical method of producing a grounded printed circuit with a minimum amount of labor and time.

It is still another object of this invention to provide a photomechanical method of producing a grounded printed circuit which is economical to produce and which utilizes conventional currently available equipment that lends itself to standard mass-producing manufacturing techniques.

For a better understanding of the present invention together with other and further objects thereof, reference is made to the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

DESCRIPTION OF THE DRAWING FIG. 1 is a plan view of a non-grounded printed circuit master utilized with the method of this invention;

FIG. 2 is a plan view of a reduced negative of the original non-grounded printed circuit utilized with the method of this invention;

FIG. 3 is a side elevational view shown partly in cross-section of the negative, the diffuser sheet and the exposed film in position with emulsions up during a step in the method of this invention;

FIG. 4 is a plan view of the blockout mask made from the reduced negative in another step in the method of this invention;

FIG. 5 is a plan view of the composite positive made by printing the reduced negative and blockout mask onto a single sheet of film in another step in the method of this invention;

FIG. 6 is a plan view of the composite print with terminals connected to the ground plane in another step in the method of this invention;

FIG. 7 is a plan view of the finished negative obtained by contact printing the composite positive in another step in the method of this invention; and

FIG. 8 is a plan -view of a completed grounded printed circuit produced by the method of this inven-' tion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In the method of the instant invention the initial step in producing a grounded printed circuit is identical to the conventional method used to produce a normal ungrounded circuit negative. As shown in FIG. 1 of the drawing, a draftsman working from a sketch prepared by an engineer prepares a circuit master 10 by using pre-cut or pre-printed press-on symbols 12 to lay out the circuit on a sheet of plastic, such as matte Mylar 14. This circuit master 10 is then sent to a photo lab where it is reduced to the desired finished size on a lithographic film by the use of a conventional process camera.

The resulting negative 16 as best shown in FIG. 2 is then opaqued when dry to eliminate any pinholes which may be formed therein. The next step in the method of this invention is to punch the negative 16 at 18 with any suitable two-hole registration punch so that this negative 16 when utilized in conjunction with other film may be set up in proper alignment therewith. This alignment may be accomplished with the use of Carlson V4 inch pins; however, any other system that accepts pins that can be used in a vacuum frame will also suffice.

Reference is now made to FIG. 3 of the drawing in which the position of negative 16 is reversed so that its emulsion surface 19 is facing upward. With the negative 16 in this position it is pinned to any suitable translucent sheet 20 such as double matte Mylar, 0.007 inch thick, which has also been punched at 22 for alignment purposes. A sheet of stable base film 24 which has been similarly punched at 26 and also having its emulsion surface facing upward 28 is now pinned to the other two sheets 16 and 20. What we now have shown in FIG. 3 of the drawing are three sheets of film pinned together; a processed circuit negative 16 with emulsion surface 19 up, an unexposed sheet of film 24 underneath with emulsion surface 28 up and sandwiched between these two sheets 16 and 24 any suitable diffuser 20. It should be noted that it is necessary for the negative 16 to be printed upside down, that is having its emulsion surface 19 up, so that in the final composite print set described hereinbelow the negative 16 and the film 24 which is now being produced will be printed to the positive state with emulsion to emulsion contact. Still referring to FIG. 3, these three sheets 16, 20 and 24 are now placed in any suitable clamp or vacuum frame (not shown) and are overexposed by light source 30 by a factor of some 5-10 times. It has been found that a factor of about ten times with a developing time of 2% minutes has produced extremely satisfactory results.

The exposed film 24 is now developed in any conventional infectious type lithographic developer in order to produce the spread image on the blackout mask 31 shown in FIG. 4. The spreading of the image is usually about three times the width of a wire line 33 shown in FIG; 2; that is, if the wire line 33 is l/l6 inch thick the spread of wire line 33 of mask 31 will be approximately 3/16 inch thick. This mask 31 upon drying is opaqued so that any pinholes therein will be removed and it is fully prepared to produce the composite posi tive print 32 shown in FIG. 5.

Referring to FIG. 5 of the drawing, the composite positive 32 is produced by printing a punched and pinned sheet of unexposed film by contact, at normal exposure time, first to the original negative 16 emulsion-to-emulsion and then to the spread mask 31 emulsion-to-emulsion. This procedure preferably takes place under a vacuum to assure direct contact between the surfaces. The positive 32 best shown in FIG. 5 is now developed in a conventional manner, washed, dried and opaqued.

FIG. 6 shows the next step of the instant invention which is the process of connecting the terminals which require grounding by either tape or liquid opaque at 34. When the grounding has been completed the composite positive 32 is contacted back to the negative state as shown in FIG. 7 of the drawing.

This composite negative 32 shown in FIG. 7 can now be sent to the etching shop where utilizing the conventional method of producing an ungrounded printed circuit, the negative 32 is placed in intimate contact with a copper-clad plastic board which has been coated with a photo resist. The negative and board are then exposed in a conventional manner to an ultraviolet light source which hardens the resist wherever the clear areas appear on the negative. The unhardened resist is then developed away leaving etchant resistant copper where the circuit terminals 38, the wires 40 and the ground plane 42 are located and unprotected copper in the other areas. The board is then etched and the hardened resist is removed thereby producing the grounded printed circuit 44 as best shown in FIG. 8 of the drawmg.

The method of this invention will save substantial man hours in producing a grounded printed circuit 44. Furthermore, a much neater job is accomplished by this method thereby enhancing the appearance of the board and allowing for a high saturation of components as compared to the old method. Also, fewer mistakes can be made with the instant method than by the prior art taping method.

Although this invention has been described with reference to a particular embodiment, it will be understood to those skilled in the art that this invention is also capable of a variety of alternative embodiments within the spirit and scope of the appended claims.

I claim:

1. A photomechanical method of producing a grounded printed circuit comprising the steps of:

a. preparing a printed circuit master by affixing symbols in a predetermined pattern to a sheet of plastic;

b. photographing said printed circuit master and reducing the resultant photograph to a desired size;

c. producing a negative from said desired size photograph;

d. forming a laminated structure by sandwiching and aligning a translucent sheet between and in contact with a top layer made up of said negative and a botton layer made up of a stable base film such that said negative and said stable base film have their respective emulsion surfaces directed upward;

e. fixing said laminated structure with respect to a light source directly thereabove and overexposing said laminated structure by said light source by a factor of about 5l0 times;

f. developing said stable base film, thereby producing a positive blockout mask having a spread image thereon;

g. producing a composite positive of said negative and said positive blockout mask by printing a sheet of unexposed film in emulsion to emulsion contact at normal exposure time, first to said negative and then to said positive blockout mask;

h. grounding said symbols in said composite positive by connecting preselected symbols to each other and to a ground plane;

i. producing a grounded composite negative from said grounded composite positive; and

j. forming said grounded printed circuit from said grounded composite negative by affixing a conductive material to the upper surface of a plastic board, coating said conductive surface of said plastic board with a photoresist, placing said grounded composite negative in intimate contact with said coated conductive surface of said plastic board, ex-

posing said grounded composite negative and said board to an ultraviolet light source to harden said photoresist wherever clear areas of said grounded composite negative appear, removing said unhard' ened resist thereby leaving areas of conductive surface having photo-resist thereon and areas of conductive surfaces having no photoresist thereon, removing said conductive surfaces having no photoresist thereon thereby exposing said plastic board therebeneath, and removing said photo-resist from said remaining conductive surfaces. 

